Zilog Z08470 - Manual (Page 108)

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UM008101-0601 Direct Memory Access

Figure 38. Polling for a Service Request Bit
PROGRAMMING
Overview
The DMA must be programmed before use. Its control registers have no
useful default values at power-up. In addition, commands are frequently
written to the DMA after the initial power-up programming sets basic
DMA operating modes; this is most commonly done within service
routines for purposes such as reading status, changing starting addresses,
and reenabling both interrupt and bus-request logic after a block transfer
or search.
The DMA has two primary states that can be set: (1) an enabled state, in
which the DMA gains control of the system buses and directs data
transfers between ports or data searching from a single port; and (2) a
disabled state, in which the DMA initiates neither bus requests nor data
transfers. Table 13 describes these states and their substates in detail.
When the DMA is powered-up or reset by any means, it is automatically
placed into the disabled state. Program commands can be written to it by
the CPU in the enable/inactive state, but this automatically puts the DMA
into the disabled state, which is maintained until an ENABLE DMA
command is written by the CPU to the DMA’s Write Register 6 (WR6).
In the Z80 Family, the DMA normally exists as a peripheral device in
system I/O space. Its Chip Enable (CE) signal is decoded from the lower
byte of the address bus for this purpose and all control bytes and status
bytes are written to and read from the same I/O port address, using an
output instruction such as OTIR (in the Z80 CPU).
It is possible to use the DMA in memory mapped I/O structures, but this
involves some external logic, which is explained in the “Applications”